netdev - WARN_ON() in netconsole with PREEMPT

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Date:   Sun, 11 Nov 2018 20:13:40 +0100
From:   Oleksandr Natalenko <oleksandr@...alenko.name>
To:     "David S. Miller" <davem@...emloft.net>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Eric Dumazet <edumazet@...gle.com>,
        Dave Jones <davej@...emonkey.org.uk>, netdev@...r.kernel.org,
        linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: WARN_ON() in netconsole with PREEMPT_RT

Hi.

I was just curious about PREEMPT_RT and decided to give it a (small) 
shot on my laptop. As a safety measure, I've enabled netconsole to catch 
all the weird stuff that can pop up, and immediately it indeed did… in 
the netconsole code itself (irony):

===
[   64.018949] WARNING: CPU: 0 PID: 1204 at net/core/netpoll.c:372 
netpoll_send_udp+0x3e8/0x3ef
…
[   64.019057] CPU: 0 PID: 1204 Comm: bash Not tainted 4.19.0-ig1 #1
[   64.019058] Hardware name: Dell Inc.          Vostro 3360/0F5DWF, 
BIOS A18 09/25/2013
[   64.019063] RIP: 0010:netpoll_send_udp+0x3e8/0x3ef                    
                                                              [   
64.019066] Code: dd ff ff 41 ba 86 dd ff ff 49 2b 96 d0 00 00 00 66 41 
89 96 c6 00 00 00 66 44 89 48 0c 66 45 89 96 c0 00 00 00 e9
  93 fe ff ff <0f> 0b e9 40 fc ff ff 0f 1f 44 00 00 53 0f b7 57 42 48 89 
fb 48 8b
[   64.019068] RSP: 0018:ffffaa6743f87c98 EFLAGS: 00010202
[   64.019070] RAX: 0000000000000292 RBX: 0000000000000014 RCX: 
0000000000000028
[   64.019072] RDX: 0000000000000014 RSI: ffffffffb1e6a300 RDI: 
ffff95973d905668
[   64.019073] RBP: ffffaa6743f87cd8 R08: ffffffffc11f6ba0 R09: 
0000000000000000
[   64.019075] R10: 0000000000000008 R11: 0000000000000000 R12: 
00000000000003e8
[   64.019076] R13: ffff95973d905668 R14: 0000000000000014 R15: 
ffff95973d905668
[   64.019079] FS:  00007f2f8b809b80(0000) GS:ffff95976f000000(0000) 
knlGS:0000000000000000
[   64.019081] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   64.019083] CR2: 000055b8e126fbc8 CR3: 0000000415440006 CR4: 
00000000001606f0
[   64.019084] Call Trace:
[   64.019098]  write_msg+0xd1/0xe0 [netconsole]
[   64.019107]  console_unlock.part.6+0x55b/0x5a0
[   64.019115]  ? _raw_spin_unlock_irqrestore+0x20/0x60
[   64.019120]  vprintk_emit+0x16a/0x1a0
[   64.019125]  printk_emit+0x44/0x5b
[   64.019130]  ? _raw_spin_trylock+0x13/0x80
[   64.019134]  devkmsg_write.cold.16+0x21/0x4e
[   64.019140]  __vfs_write+0x136/0x1a0
[   64.019145]  vfs_write+0xa9/0x1a0
[   64.019149]  ksys_write+0x52/0xc0
[   64.019156]  do_syscall_64+0x5b/0x170
[   64.019160]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
…
===

This is a v4.19.1-rt3-based kernel.

The WARN_ON() is:

362 void netpoll_send_udp(struct netpoll *np, const char *msg, int len)
363 {
…
372     WARN_ON_ONCE(!irqs_disabled());
…

If that matters, I have "threadirqs" passed to the kernel.

Netconsole seems to work even after this warning. Is this OK/expected?

Thanks.

-- 
   Oleksandr Natalenko (post-factum)